This invention relates to the measurement of tissue oedema and, in particular, lymphoedema using bioelectrical impedance analysis.
Measurement of extracellular tissue fluid may be of importance in a range of situations. This is particularly so in the case of lymphoedema which is a condition characterised by excess protein and oedema in the tissues as a result of reduced lymphatic transport capacity and/or reduced tissue proteolytic capacity in the presence of a normal lymphatic load. Lymphoedema normally occurs in a limb and may cause pain, scarring and reduced limb function. The condition is incurable, progressive, often disfiguring and physically disabling. Its course, however, can be arrested or slowed by intervention using physical therapy, compression bandaging, massage and other physical techniques.
Acquired or secondary lymphoedema is caused by damaged or blocked lymphatic vessels. The commonest inciting events are surgery and/or radiotherapy. However, onset of lymphoedema is unpredictable and may develop within days of its cause or at any time during a period of many years after that cause.
There is a need for an accurate and effective technique to detect the onset of lymphoedema, assess its severity and monitor its response to treatment. The simplest known technique involves measurement of limb circumferences and comparison with a paired unaffected limb. A further technique is available by way of immersion of the affected part and measurement of displaced liquid with subsequent comparison against the result of the same measurement performed on an unaffected limb.
It is also known to use multiple frequency bioelectrical impedance analysis (MFBIA) to assess lymphoedema (Watanabe et al., 1989, Lymphology 22:85). The authors noted that when a low frequency voltage is applied to tissue, the impedance of the cell membrane is substantial. With increased frequency, the impedance of the cell membrane decreases and current is able to flow through both extracellular and intracellular fluids. The results obtained by Watanabe et al were subject to analysis of equivalent resistivity of extracellular and intracellular fluid calculated after measurement of electrical bioimpedance at multiple frequencies. Further development of the technique was subsequently disclosed (Ward et al., 1992, European Journal of Clinical Investigation 22:751) in which MFBIA was used and the impedance at zero frequency was estimated by extrapolation. Differences were then calculated between left-hand and right-hand sides of patients for the impedance calculations 50 kHz and 0 kHz frequency. The bilateral differences in impedance between a group of controls and a group of affected patients were significant. This test relies on the use of a multifrequency bioimpedance meter and relatively complex analysis of the results to provide an indication of lymphoedema.
U.S. Pat. No 5,372,141 describes a body composition analyser that provides information in relation to body fat and ideal body weight. The analyser compares the bioimpedance of the body xe2x80x9cnetworkxe2x80x9d against a reference network of known impedance. It is, however, of little or no use in assessing tissue oedema.
U.S. Pat. No 4,947,862 discloses an analyser to determine the amount of body fat on a patient. The analyser uses a high frequency low-voltage signal in the body and measures magnitudes and phase shift of the induced signal but again is of little use in measuring tissue oedema.
It would be of advantage to provide a method for determining the presence of oedema and, in particular, lymphoedema by measurements taken at a single frequency. It would further be advantageous to produce a device for measuring bioelectrical impedance at a single frequency and analysing that measurement to produce an indicator of the presence of oedema.
It is an object of the present invention to overcome or ameliorate one or more of the difficulties of known methods used to assess tissue oedema and, in particular, lymphoedema.
In one form, although it need not be the only or broadest form, the invention resides in a method of assessing tissue oedema comprising the steps of:
performing a first measurement of bioelectrical impedance of a first anatomical region in a subject at a single low frequency alternating current;
performing a second measurement of bioelectrical impedance of a second anatomical region in the same subject at the same low frequency alternating current; and
analysing the two measurements to obtain an indication of the presence of tissue oedema.
The first anatomical region and second anatomical region may be paired with at least one of the anatomical regions unaffected by tissue oedema.
Alternatively, the first and second anatomical regions may be dissimilar with at least one of the anatomical regions unaffected by tissue oedema.
The first anatomical region and the second anatomical region may be the same region with the first and second measurements separated in time. The anatomical regions may be limbs or parts of limbs.
The low frequency is preferably in the range of 5 to 20 kHz. More suitably, the range is 10 to 15 kHz. Most preferably, the measurements are made at 10 kHz.
The analysis may include the step of dividing the lesser result of the two measurements into the greater result of the two measurements to obtain a product or quotient. The results of the two measurements may further include the steps of applying a correcting factor or term to the product and deriving an indication of tissue oedema.
The step of analysing the two measurements may be conducted according to the algorithm   F  =                    Z        h                    Z        l              -    cf  
where:
F is an indication of the presence of tissue oedema;
Zh is the greater bioelectrical measurement;
Zl is the lesser bioelectrical measurement; and
cf is a correcting factor.
The method may include the step of establishing xe2x80x9ccfxe2x80x9d. Establishing xe2x80x9ccfxe2x80x9d may include the step of establishing a ratio of the bioelectrical impedance of a first anatomical region of at least one subject unaffected by tissue oedema compared to the bioelectrical impedance of a second anatomical region of that subject wherein the first and second anatomical regions of the at least one unaffected subject are paired with the first and second anatomical regions of the subject being assessed for tissue oedema.
When analysing the results of two measurements obtained on paired limbs, the correcting factor may suitably be 1.066.
Alternatively, the step of analysing the two measurements may be conducted according to the algorithm   F  =            cf      1        -                  Z        l                    Z        h            
where:
F is an indication of the presence of tissue oedema;
cf1=a correcting factor;
Zl is the lesser bioelectrical impedance measurement; and
Zh is the greater bioelectrical impedance measurement.
When analysing the results of two paired limbs cf1 may be 0.862.
The indication of tissue oedema may be displayed by the step of representing the indication as a position on a scale.
In an alternate form, the invention resides in an apparatus for determining the presence of tissue oedema, including:
current means for applying an alternating current to an anatomical region at a single frequency;
monitoring means to monitor the bioelectrical impedance of said region and produce signals characteristic of bioimpedance; and
analysis means to analyse the signals indicative of bioimpedance to provide an indication of tissue oedema.
The current means may suitably be a proximal electrode and distal electrode in electrical connection with a power source. The monitoring means is suitably a first connection and second connection for location on or near the anatomical region. Preferably, the monitoring means includes display means to display the signals indicative of bioimpedance.
Suitably; the analysis means is at least one processing means programmed to perform analysis of data to provide an indication of the presence of tissue oedema.
The analysis means may be programmed to analyse data according to the algorithm   F  =                    Z        h                    Z        l              -    cf  
where:
F is an indication of the presence of tissue oedema;
Zh is a greater bioelectrical impedance measurement obtained from a first anatomical region;
Zl is a lower bioelectrical impedance measurement obtained from a second anatomical region; and
cf is a correcting factor.
Suitably, cf may equal 1.066 when the first and second anatomical regions of a subject undergoing assessment for tissue oedema are paired limbs.
The apparatus preferably includes means for recording bioimpedance in anatomical regions of the same subject simultaneously.
Preferably, said means includes duplicated electrodes and connections.